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Light-Dependent Reactions

In the light-dependent reactions, energy from sunlight is absorbed by the molecule chlorophyll and converted into stored chemical energy in the form of the electron carrier molecule NADPH (nicotinamide adenine dinucleotide phosphate) and the energy currency molecule ATP (adenosine triphosphate). The light-dependent reactions take place in the thylakoid membranes in the granum (stack of thylakoids) in the chloroplast.

Photosystems

The actual step that converts light energy into chemical energy takes place in a multi-protein complex called a photosystem. Two types of photosystems are embedded in the thylakoid membrane: photosystem II ( PSII) and photosystem I (PSI). Each photosystem plays a key role in capturing the energy from sunlight by exciting electrons. These energized electrons are transported by "energy carrier" molecules and they power the light-independent reactions.

The two photosystems differ on the basis of what they oxidize (i.e., the source of the low-energy electron supply) and what they reduce (i.e., the place to which they deliver their energized electrons). Each photosystem responds to a different wavelength of light.

Light-Independent Reactions

In the light-independent reactions or Calvin Cycle, the energized electrons from the light-dependent reactions provides energy to assemble carbohydrates from carbon dioxide molecules. The light-independent reactions are sometimes called the Calvin Cycle because carbons are constructed into carbohydrate molecules in a cycle of chemical processes.

Although the light-independent reactions do not use light as a reactant, they require the products of the light-dependent reactions to function. The light-independent molecules depend on the energy carrier molecules to drive the construction of new carbohydrate molecules. After the energy is transferred, the energy carrier molecules return tot he light-dependent reactions to obtain more energized electrons. In addition, several enzymes of the light-independent reactions are activated by light.

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